Capacitor switching arrangement for regulating power output of heating magnetron

ABSTRACT

In a microwave heating apparatus comprising a magnetron, a plurality of capacitors are provided in the high tension rectifier circuit of the magnetron to vary the power output from the magnetron, and are changed over by the same number of switches as the number of the capacitors.

United States Patent ag 'fl 51 Aug. 15, 1972 [54] CAPACITOR SWITCHING ARRANGEMENT FOR REGULATING [56] References Cited POWER OUTPUT OF HEATING M AGNETRON UNITED STATES PATENTS 72 Inventor; Tadayasu Otaguro, Kawasaki, Japan 3,396,342 8/1968 Feinberg ..328/253 X 3,470,942 10/1969 Fukada et a1. "219, 10.55 X [73] Ass'gnee= The General Corporation, 3,591,826 7/1971 Valles ..219/10.55

Kawasaki-shi, Japan [22] Filed: Dec. 16, 1970 Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm [21] Appl 98735 Att0rneyLint0n & Linton [30] Foreign Application Priority Data [57] ABSTRACT DEC. 18, 1969 Japan ..44/ 120376 In a microwave heating apparatus omprisi a ma netron, a plurality of capacitors are provided in the [52] US. Cl. .-33l/86, 219/ 10.55, 315/239, hi h tension rectifier circuit of the magnetron to vary 323/61, 328/253, 328/263, 331/182, 331/ 8 the power output from the magnetron, and are [51] Int. Cl. ..H03b 9/10, H05b 9/00 Changed over by the same number f Switches as the [58] Field of Search ..33l/86, 87, 182, 185; 328/253,

number of the capacitors.

6 Claims, 5 Drawing Figures PATENTEDms 15 m2 SHEET 1 0F 2 INVENTOR.

W1 dd 1' ,72740 PATENTEDAUB 15 I972 sum 2 or 2 FiG.5

5 MEMORY RELAY T R E M T m/ M L F- R l F F W O M.. 1% W 0B 0 I NVENTOR.

CAPACITOR SWITCHING ARRANGEMENT FOR REGULATING POWER OUTPUT OF HEATING MAGNETRON SUMMARY OF THE INVENTION ferent power levels, the microwave output from a magl netron that produces microwave energy in a dielectric heating apparatus.

Where such a microwave heating apparatus is applied, for example, to an electronic cooking of food by dielectric heating, there is a need to change the power output in accordance with the varieties of the food to be cooked. However, changing the power output level of a magnetron device involved technical difficulties, since the a.c. supply voltage to the heating apparatus cannot be varied because of the necessity of maintaining a constant rated d.c. supply voltage to the magnetron for the stabilization of the oscillation thereof. As is well known, a variation of the dc. supply voltage to the magnetron from the rated value causes the oscillation frequency to become unstable, and further variation results in the interruption of the oscillation. In addition, control of the dc. power as by providing on a step-up transformer of the high tension circuit a plurality of secondary windings having the same turn ratio but having different inductances for varying the impedance of the high tension d.c. circuit by switching between these secondary windings is also difficult because of the high tension. Provision of a plurality of such high tension windings is not practical, and however, the switching of them will be even prohibitive.

Therefore, it is a general object of the invention to provide an output switching device for a microwave heating apparatus incorporating a magnetron, which permits a ready switching of the power output level with a simple operation without recourse to variation of the dc. high tension supplied to the magnetron.

It is specific object of the invention to provide an output switching device for home appliance microwave cooking apparatus.

It is another object of the invention to provide an output switching device for microwave cooking apparatus which is simple in construction and inexpensive to manufacture.

It is a further object of the invention to provide an output switching device for a microwave heating apparatus which affords protection to the switching elements used from transient current upon switching.

In accordance with the invention, there is provided a step-up transformer for supplying a secondary high tension which is rectified by a rectifier to feed a magnetron. A plurality of capacitors are provided in the high tension circuit, that is, between the high tension winding of the transformer, and the rectifier and are arranged to be alternatively or selectively connected into circuit by a change-over switch for varying the effective impedance in the dc. high tension circuit. It is another feature of the invention to effect the switching of the capacitors only when the primary circuit of the transformer is opened. At this end, the primary circuit of the transformer includes an electromagnetic contactor which is normally energized to close the primary circuit and which is deenergized by an auxilliary switch connected in series therewith which is interlocked by suitable means, e.g. by mechanical means, with the operation of the change-over switch in a manner to be operated prior to the operation of the change-over switch. This avoids high current transients and consequent damage to the contacts of the change-over switch which would otherwise occur in the secondary circuit of the transformer because of the high in 0 ductance therein when the capacitors are switched BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a wiring diagram of a microwave cooking apparatus according to one embodiment of the invention,

FIG. 2 is wiring diagram of another embodiment of the invention,

FIG. 3 is a wiring diagram of a further embodiment of the invention,

FIG. 4 is a perspective view, partially broken away, of an example of a change-over switch used to change capacitance, and

FIG. 5 is a block diagram of an electrical circuit which is equivalent in function to the change-over switch of FIG. 4.

Referring to the drawings, and particularly to FIG. 1, there is shown a basic circuit arrangement of a microwave cooking apparatus constructed in accordance with the invention. The apparatus includes a step-up transformer 2 having its primary side connected with a plug 1 through a pair of contacts 31 and 32 of an electromagnetic contactor 3. The plug l is adapted to be used with a convenience outlet. Across the wires leading to the plug 1 are connected a pushbutton start switch 4, the coil of the contactor 3 and a normally closed auxilliary switch 5 in series, the start switch 4 being operated to energize the contactor 3 when cooking is to be initiated. The auxilliary switch 5 is interlocked with a capacitance change-over switch to be described later in a suitable manner, e.g. mechanically. Across the secondary winding of the transfonner 2 are connected a rectifier 9 and a capacitor 6 in series. A magnetron 10 is connected in parallel with the rectifier 9, with the anode of the magnetron 10 being connected with .the cathode of the rectifier 9. The magnetron 10 is of the direct heated cathode or filament type and the filament current is supplied by a separate transformer 1111. It should be obvious that the magnetron 10 includes a field means (not shown) to establish a steady magnetic field to sustain oscillation. The capacitor 6 is shunted by another capacitor 7 connected in series with a capacitance change-over switch 8. It will be noted that the magnetron 10 is fed by a voltage doubler circuit.

The embodiment shown in FIG. 2 is similar to the arrangement of FIG. 1 except that the capacitors 6 and 7 are connected in series rather than being in parallel connection as shown in FIG. 1, with the capacitor 7 being shunted by the change-over switch 8.

In FIG. 3, the single diode rectifier 9 of FIG. 1 is replaced by a full-wave rectifier.

In operation, the plug 1 is inserted into a convenience outlet, and the start switch 4 is depressed to energize the coil of the contactor 3 through the normally closed switch 5, whereby the pair of contacts 31 and 32 are closed. A self-holding circuit is established for the contactor 3 by closure of the contacts 31, thereby closing and feeding the primary circuit of the transformer 2. When the capacitance changeover switch 8 is open as shown, the capacitor 7 remains without connection with the circuit, so that in the arrangement of FIGS. 1 and 3, the secondary winding of the transformer 2 and the capacitor 6 form a relatively high impedance to supply the magnetron 10 with a rectified power of a relatively low level. As a consequence, the magnetron 10 is operated at a lower output level. In order to operate the magnetron at its higher output level, the change-over switch 8 is operated to connect the capacitor 7 in parallel with the capacitor 6 to thereby decrease the impedance of the d.c. high tension circuit. As mentioned previously, the change-over switch 8 is interlocked with the auxilliary switch in a suitable manner, e.g. mechanically, such that the auxilliary switch 5 is momentarily opened prior to the operation of the change-over switch 8. (Such interlocking means will be more fully described later.) As a result of this, the self-holding circuit of the contactor 3 is interrupted, and hence the contactor 3 is deenergized to open the pair of its contacts 31 and 32 connected in the primary circuit of the transformer 2. Now the change-over switch 8 is closed and the start switch 4 is depressed again, and since the auxilliary switch 5 has resumed its normal position by this time, the transformer 2 is fed with power again, with the capacitors 6 and 7 connected in parallel in its secondary circuit to reduce the resulting impedance and to operate the magnetron at its higher output level.

When the operation of the magnetron is to be changed from its higher output level to its lower output level, the change-over switch 8 is operated to disconnect the capacitor 7 from the parallel connection with the capacitor 6. Again, the operation of the changeover switch is preceded by the momentary opening of the auxilliary switch 5 to deenergize the contactor 3 so that the transformer 2 may be disconnected from the supply line when the change-over switch 8 is operated. Thereafter, the start switch 4 is depressed again to operate the magnetron 10 at its lower output level.

It will be appreciated that in the arrangement of FIG. 2, the magnetron 10 also operates at its lower output level when the change-over switch 8 is open since at this time the capacitors 6 and 7 are connected in series to present a reduced composite capacitance as compared with the capacitance presented by the capacitor 6 alone when the change-over switch 8 is closed.

FIG. 4 shows a specific embodiment of the capacitance change-over switch 8. The change-over switch shown comprises a switch casing 18, in the top wall of which are loosely fitted a pair of spaced levers 16 and 17 for vertical movement therein. The levers 16 and 17 are provided with push-buttons l2 and 13, respectively, at their top end and have projections, 14 and 15 on their lower portions located within the casing 18. A pair of return springs 19 and 20 extend between the push-buttons 12, 13 and the top wall of the casing 18. The casing 18 includes a pair of opposite side walls 21 in which elongate apertures are formed at the same level for slidably receiving a transverse plate 22. The transverse plate 22 is urged in one direction (to the left in the example shown) by a return spring 23 that extends between one of the side walls 21 and a shoulder formed on the transverse plate. The transverse plate 22 has a substantial width vertically and has a pair of ramp surfaces 24 and 25 formed on its top portion which cooperate with the projections 14 and 15 on the levers 16 and 17, respectively, and also has a pair of notches 26 and 27 formed therein at positions directly below the ramp surfaces 24 and 25, respectively. The casing 18 further includes a movable contact member 28 of an elastic or flexible material and a fixed contact 29, both suitably secured to the casing. An operating member 30 is connected with the transverse plate 22 and has a narrow slot in which the free end of the movable contact member 28 is fitted. On the bottom wall of the easing 18 is pivotally mounted another movable contact member 33 by a fulcrum member 34 which also provides an electrical connection with the movable contact member 33. The member 33 is adapted to undergo rocking motion about the fulcrum 34 by cooperation with the respective free ends of the levers 16 and 17, and at the extreme positions of such rocking motion, its opposite ends are brought into abutting relationship and electrical contact with a pair of fixed contacts 35 and 36, respectively, which are mounted on the inner surface of the bottom wall of the casing 18.

In the position shown in which the push-button 12 is depressed, the projection 14 on the lever 16 is locked in the notch 26 after having moved the transverse plate 22 to the right, as seen in FIG. 4, by sliding engagement with the ramp surface 24 thereon with the transverse plate 22 being moved back a distance corresponding to the depth of the notch 26 by the action of the return spring 23 after the projection 14 has moved past the ramp surface 24 downwardly. In this position, the operating member 30 has returned to its normal position in which it restores the movable contact 28 into electrical contact with the fixed contact 29 again. The lower end of the lever 16 has depressed the movable contact member 33 into contact with the fixed contact 35 on the bottom wall of the casing 18. If the push-button 13 is now depressed, the projection 15 on the lever 17 slides downwardly on the ramp surface 25, thereby moving the transverse plate 22 to the right again. As such movement of the transverse plate 22 commences, the movable contact member 28 is yieldably flexed away from the its cooperating contact 29 by the action of the operating member 30 that is carried by the transverse plate 22, thereby interrupting the circuit between the contacts 28 and 29. The rightward movement of the transverse plate 22 releases the projection 14 on the lever 16 from its locked position within the notch 26, and therefore the lever 16 returns to its upper position under the action of the return spring 19. As a consequence, the movable contact member 33 which has been held in abutting relationship with the contact 35 by the free end of the lever 16 is freed, and now is depressed by the free end of the other lever 17 into abutting relationship with the contact 36. By this time, the projection has moved downwardly past the ramp surface by driving the transverse plate 22 to the right, whereupon the latter is urged back by the return spring 23 to lock the projection 15 in the notch 27. At the same time, the operating member returns to its normal position in which the movable contact member 28 resumes its electrical contact with the fixed contact 29.

As will be noted, by alternate depression of the pushbuttons 12 and 13, the contact member 34 is selectively connected with either fixed contact 35 or 36, and such switching operation is preceded by the traversing motion of the transverse plate 22 which provides a momentary interruption between the contacts 28 and 29. Thus, it will be appreciated that either one of the fixed contacts 35 and 36 may be used in combination with the contact 34 to serve as the capacitance change-over switch 8 described above in connection with FIGS. 1 to 3, and the pair of contacts 28 and 29 may be used as the auxilliary switch 5 that is interlocked with the changeover switch 8.

The capacitance change-over switch 8 may comprise a switching circuit as shown in FIG. 5. In order to interrupt the holding circuit of the contactor 3 momentarily, a relay X, is connected to both the binary.one and zero outputs of a flipflop FF, which receives a set input through a push-button switch B, and a reset" input through another push-button switch 8,. These inputs are also fed to a timer T, the output of which operates another flipflop FF, having its output connected with another relay X The relay X, is of the type which instantaneously operates in response to a variation of an input thereto and returns its associated contacts to their normal position after a predetermined time delay. The relay X, has its contacts connected in the holding circuit of the contactor 3, which the relay X has its contacts connected so as to connect the capacitor 7 in circuit with the capacitor 6. The switches B, and B are connected with a suitable potential source, and are selectively operated to closewhen both capacitors 6 and 7 or capacitor 6 alone is required in the dc. high tension circuit. The timer provides a time delay irrespective of which of the switches B, and B is closed before conveying a corresponding signal to the flipflop FF On the other hand, the relay X, is immediately operated, without a time delay, and hence prior to the operation of the relay X Having described the invention, what is claimed is:

1. An output switching device for a microwave heating apparatus including a magnetron, a step-up transformer having a high tension secondary winding, and a rectifier for supplying a rectified high tension to the magnetron; said device comprising a plurality of capacitors, and a capacitance change-over switch for connecting selected one or ones of the capacitors in the high tension rectifier circuit on the secondary side of said transformer.

2. An output switching device according to claim 1, in which said plurality of capacitors are connected in series relationship to each other in the high tension reflctifier circuits an further including capacitance c ange-over switch or shortcircuiting se ected one or ones of the capacitors.

3. An output switching device according to claim 1, in which the capacitors are connected in parallel relationship to each other by said capacitance change-over switch.

4. An output switching device according to claim 1, further including an electromagnetic contactor having its contacts connected in the primary circuit of the transformer, and a normally closed auxilliary switch connected in the self-holding circuit of the contactor, said auxilliary switch being interlocked with the operation of the capacitance change-over switch in a manner such that the auxilliary switch is opened before the capacitance change-over switch operators.

5. An output switching device according to claim 4, in which said capacitance change-over switch comprises a push-button switch having detent means which includes a transverse plate capable of transversing motion in response to the operation of the push-button, said transverse plate carrying an operating member which opens the auxilliary switch prior to the operation of the capacitance change-over switch.

6. An output switching device according to claim 4, further including a first relay responsive to an operating instruction to said capacitance change-over switch for operating the auxilliary switch, a timer energized simultaneously with the first relay for providing a time delay of a predetermined length, and a second relay responsive to the output from the timer for operating said capacitance change-over switch. 

1. An output switching device for a microwave heating apparatus including a magnetron, a step-up transformer having a high tension secondary winding, and a rectifier for supplying a rectified high tension to the magnetron; said device comprising a plurality of capacitors, and a capacitance change-over switch for connecting selected one or ones of the capacitors in the high tension rectifier circuit on the secondary side of said transformer.
 2. An output switching device according to claim 1, in which said plurality of capacitors are connected in series relationship to each other in the high tension rectifier circuits, and further including a capacitance change-over switch for shortcircuiting selected one or ones of the capacitors.
 3. An output switching device according to claim 1, in which the capacitors are connected in parallel relationship to each other by said capacitance change-over switch.
 4. An output switching device according to claim 1, further including an electromagnetic contactor having its contacts connected in the primary circuit of the transformer, and a normally closed auxilliary switch connected in the self-holding circuit of the contactor, said auxilliary switch being interlocked with the operation of the capacitance change-over switch in a manner such that the auxilliary switch is opened before the capacitance change-over switch operators.
 5. An output switching device according to claim 4, in which said capacitance change-over switch comprises a push-button switch having detent means which includes a transverse plate capable of transversing motion in response to the operation of the push-button, said transverse plate carrying an operating member which opens the auxilliary switch prior to the operation of the capacitance change-over switch.
 6. An output switching device according to claim 4, further including a first relay responsive to an operating instruction to said capacitance change-over switch for operating the auxilliary switch, a timer energized simultaneously with the first relay for providing a time delay of a predetermined length, and a second relay responsive to the output from the timer for operating said capacitance change-over switch. 